Hi Brian, Thanks for review. > From: Brian Norris [mailto:briannorris@xxxxxxxxxxxx] > Sent: Tuesday, October 25, 2016 6:22 AM > To: Amitkumar Karwar > Cc: linux-wireless@xxxxxxxxxxxxxxx; Cathy Luo; Nishant Sarmukadam; > rajatja@xxxxxxxxxx; Xinming Hu; abhishekbh@xxxxxxxxxx; Dmitry Torokhov > Subject: Re: [PATCH v4 1/3] mwifiex: reset card->adapter during device > unregister > > Hi Amit, > > On Thu, Oct 20, 2016 at 01:11:31PM +0000, Amitkumar Karwar wrote: > > > From: Brian Norris [mailto:briannorris@xxxxxxxxxxxx] > > > Sent: Tuesday, October 11, 2016 5:53 AM > > > To: Amitkumar Karwar > > > Cc: linux-wireless@xxxxxxxxxxxxxxx; Cathy Luo; Nishant Sarmukadam; > > > rajatja@xxxxxxxxxx; Xinming Hu; abhishekbh@xxxxxxxxxx; Dmitry > > > Torokhov > > > Subject: Re: [PATCH v4 1/3] mwifiex: reset card->adapter during > > > device unregister > > > > > > On Mon, Oct 10, 2016 at 01:53:32PM -0700, Brian Norris wrote: > > > > On Thu, Oct 06, 2016 at 11:36:24PM +0530, Amitkumar Karwar wrote: > > > > > From: Xinming Hu <huxm@xxxxxxxxxxx> > > > > > > > > > > card->adapter gets initialized during device registration. > > > > > As it's not cleared, we may end up accessing invalid memory in > > > > > some corner cases. This patch fixes the problem. > > > > > > > > > > Signed-off-by: Xinming Hu <huxm@xxxxxxxxxxx> > > > > > Signed-off-by: Amitkumar Karwar <akarwar@xxxxxxxxxxx> > > > > > --- > > > > > v4: Same as v1, v2, v3 > > > > > --- > > > > > drivers/net/wireless/marvell/mwifiex/pcie.c | 1 + > > > > > drivers/net/wireless/marvell/mwifiex/sdio.c | 1 + > > > > > 2 files changed, 2 insertions(+) > > > > > > > > > > diff --git a/drivers/net/wireless/marvell/mwifiex/pcie.c > > > > > b/drivers/net/wireless/marvell/mwifiex/pcie.c > > > > > index f1eeb73..ba9e068 100644 > > > > > --- a/drivers/net/wireless/marvell/mwifiex/pcie.c > > > > > +++ b/drivers/net/wireless/marvell/mwifiex/pcie.c > > > > > @@ -3042,6 +3042,7 @@ static void mwifiex_unregister_dev(struct > > > mwifiex_adapter *adapter) > > > > > pci_disable_msi(pdev); > > > > > } > > > > > } > > > > > + card->adapter = NULL; > > > > > } > > > > > > > > > > /* This function initializes the PCI-E host memory space, WCB > > > rings, etc. > > > > > diff --git a/drivers/net/wireless/marvell/mwifiex/sdio.c > > > > > b/drivers/net/wireless/marvell/mwifiex/sdio.c > > > > > index 8718950..4cad1c2 100644 > > > > > --- a/drivers/net/wireless/marvell/mwifiex/sdio.c > > > > > +++ b/drivers/net/wireless/marvell/mwifiex/sdio.c > > > > > @@ -2066,6 +2066,7 @@ mwifiex_unregister_dev(struct > > > > > mwifiex_adapter > > > *adapter) > > > > > struct sdio_mmc_card *card = adapter->card; > > > > > > > > > > if (adapter->card) { > > > > > + card->adapter = NULL; > > > > > sdio_claim_host(card->func); > > > > > sdio_disable_func(card->func); > > > > > sdio_release_host(card->func); > > > > > > > > As discussed on v1, I had qualms about the raciness between > > > > reads/writes of card->adapter, but I believe we: > > > > (a) can't have any command activity while writing the ->adapter > > > > field (either we're just init'ing the device, or we've disabled > > > > interrupts and are tearing it down) and > > > > (b) can't have a race between suspend()/resume() and > > > > unregister_dev(), since unregister_dev() is called from device > > > > remove() (which should not be concurrent with suspend()). > > > > > > > > Also, I thought you had the same problem in usb.c, but in fact, > > > > you fixed that ages ago here: > > > > > > > > 353d2a69ea26 mwifiex: fix issues in driver unload path for USB > > > > chipsets > > > > > > > > Would be nice if fixes were bettery synchronized across the three > > > > interface drivers you support. We seem to be discovering > > > > unnecessary divergence on a few points recently. > > > > > > > > At any rate: > > > > > > > > Reviewed-by: Brian Norris <briannorris@xxxxxxxxxxxx> > > > > Tested-by: Brian Norris <briannorris@xxxxxxxxxxxx> > > > > > > Dmitry helped me re-realize my original qualms: > > > > > > mwifiex_unregister_dev() is called in the failure path for your > > > async FW request, and so it may race with suspend(). So I retract my > Reviewed-by. > > > Sorry. > > > > Thanks for your comments. > > > > Actually description for this patch was ambiguous and incorrect. Sorry > > for that. This patch doesn't fix any race. In fact, we don't have a > > race between init and remove threads due to semaphore usage as per > > design. This patch just adds missing "card->adapter=NULL" so that when > > teardown/remove thread starts after init failure, it won't try freeing > > already freed things. > > So to be clear, you'r talking about mwifiex_fw_dpc(), which in the error > path > has: > > if (adapter->if_ops.unregister_dev) > adapter->if_ops.unregister_dev(adapter); <--- POINT A: > This is where you want to set ->adapter = NULL ... > if (init_failed) > mwifiex_free_adapter(adapter); > up(sem); <--- POINT B: This is where you release the semaphore, > which is supposed to guarantee that remove() isn't happening > return; > } > > But you *do* have a race between the above code and the remove code in > some cases. Particularly, see this: > > static void mwifiex_pcie_remove(struct pci_dev *pdev) { > struct pcie_service_card *card; > struct mwifiex_adapter *adapter; > struct mwifiex_private *priv; > > card = pci_get_drvdata(pdev); > if (!card) > return; > > adapter = card->adapter; <--- POINT C: This can execute at the > same time as unregister_dev() > if (!adapter || !adapter->priv_num) > return; > > if (user_rmmod && !adapter->mfg_mode) { #ifdef CONFIG_PM_SLEEP > if (adapter->is_suspended) > mwifiex_pcie_resume(&pdev->dev); #endif > > mwifiex_deauthenticate_all(adapter); > > priv = mwifiex_get_priv(adapter, MWIFIEX_BSS_ROLE_ANY); > > mwifiex_disable_auto_ds(priv); > > mwifiex_init_shutdown_fw(priv, MWIFIEX_FUNC_SHUTDOWN); > } > > mwifiex_remove_card(card->adapter, &add_remove_card_sem); <--- > POINT D: You only grab the semaphore here } > > So IIUC, you have a race **even here, where you claim the semaphore > should protect you** such that the adapter might be freed, but you still > can access it anywhere between C and D. i.e., you can see this: > > Thread 1 Thread 2 > (1) POINT C (retrieve adapter != > NULL) > (2) POINT A (set adapter NULL) > (3) POINT B (adapter has been freed) > (3) ....Keep accessing freed > adapter structure > (4) POINT D - acquire semaphore, > but we're too late > > Step 3 is an error, and AFAICT, that's exactly what you're trying to > solve in this patch. It essentially comes down to the same fact: you're > getting a reference to the adapter structure *without* any protection at > all. Your add_remove_card_sem is *almost* the right thing to resolve > this, but you still don't have the ordering quite right. Code between POINT C and POINT D won't come into picture for "init + reboot" scenario(Case 1 below) which we are talking here. Reason is "user_rmmod" flag will be false. Basically we have 3 teardown cases 1) System shutdown (or manually remove wifi card) -- Only mwifiex_pcie_remove() gets called. 2) User unloading the driver -- mwifiex_pcie_cleanup_module() followed by mwifiex_pcie_remove() gets called. 3) Chip isn't connected. User unloaded the driver -- Only mwifiex_pcie_cleanup_module() gets called. In case 2, we have already waited for semaphore in mwifiex_pcie_cleanup_module(). So by the time we execute mwifiex_pcie_remove(), "card->adapter" is NULL. Case 3, doesn't use "card->adapter" > > Maybe you could solve this all by acquiring the semaphore in suspend(), > remove(), shutdown(), before you ever acquire the card->adapter? If you > do that first (to fix the race), and only *then* do you submit the > $subject patch, then you might have fixed all the races in question (at > least between FW init and {suspend,resume,remove,shutdown} -- you have > plenty of other races still, both known and unknown). "add_remove_card_sem" is used only for init and teardown threads as per our design. V5 4/4 takes care of race between "init fail + suspend". Please let us know if you see any other race situation. > > [[ Also, a side note on POINT D: it's possible that even though you're > retrieving card->adapter in the argument to that function call, it's > possible the compiler will notice that this is redundant with the > retrieval at POINT C and just use that one. But even if not, there's a > window between "retrieve function argument" and "grab semaphore in > mwifiex_remove_card()". ]] I had not considered that compiler will notice this is redundant and use "card->adapter" assigned at POINT C. But this window is very small compared to the window between in "card->adapter = NULL" and releasing semaphore in other(mwifiex_fw_dpc()) thread. Even if "card->adapter" is set to NULL in mwifiex_fw_dpc() after we checked it at POINT C, we will return from mwifiex_remove_card() without grabing semaphore. mwifiex_fw_dpc() wouldn't have released the semaphore at that point of time. mwifiex_fw_dpc() will take care of performing cleanup. > > --- > > BTW I'm gonna sidetrack us here... what's up with this code, in pcie.c? > > if (!down_interruptible(&add_remove_card_sem)) > up(&add_remove_card_sem); > > I can understand that you want to grab the semaphore for the remove > code, but why do you want to immediately release it? If you release it, > that must mean that someone else is trying to get it. And they won't > notice that you're tearing down the device... I don't see any problem in releasing the semaphore here. Only other thread which can acquire this semaphore is "init" thread. "init" thread won't get called when user is unloading the driver. The purpose of immediately releasing it is we just wanted to wait until init is completed if it's running. >(Also, why aren't you > handling the interrupted case?) The cleanup performed in this function is done without touching hardware OR referring adapter/card etc. It can be done even for the interrupted case. > > > I have updated patch description in v5 series. > > > > You are right. We may have a race between init failure and suspend > > thread. I have prepared 4th patch in my v5 series to address this. > > > > > > > > I'm going to look into converting to asynchronous device probing, > > > which might remove the need for async FW request, and would > > > therefore resolve both patch 1 and 3's races without any additional > > > complicated hacks. But I'm not sure if that will satisfy all mwifiex > > > users well enough. I'll have to give it a little more thought. Any > > > thoughts from your side, Amit? > > > > > > > This is not needed. > > Yeah, I ended up deciding this really wasn't that great of a solution. > For one, you also need to do firmware reloads for your PCIe reset > handling, and this happens in parallel to any device suspend. So: > (a) I'd bet that has plenty of race conditions and > (b) that means we can't just "load the firmware synchronously once and > forget about it" > > > 4th patch in V5 series would take care of this race. > > No, patch 4 does not handle this race, and it doesn't handle the race I > point out above either. You still can get a pointer card->adapter and > then see another thread subsequently free() it out from under you. We have used spinlock in v5 4/4 to guard "card->adapter". Please help me understand if it doesn't handle the race between "init fail" + suspend. Regards, Amitkumar Karwar.
