On 09/09/2013 03:46 PM, Jerome Marchand wrote: > On 09/09/2013 03:21 PM, Dan Carpenter wrote: >> On Mon, Sep 09, 2013 at 03:49:42PM +0300, Sergey Senozhatsky wrote: >>>>> Calling handle_pending_slot_free() for every RW operation may >>>>> cause unneccessary slot_free_lock locking, because most likely >>>>> process will see NULL slot_free_rq. handle_pending_slot_free() >>>>> only when current detects that slot_free_rq is not NULL. >>>>> >>>>> v2: protect handle_pending_slot_free() with zram rw_lock. >>>>> >>>> >>>> zram->slot_free_lock protects zram->slot_free_rq but shouldn't the zram >>>> rw_lock be wrapped around the whole operation like the original code >>>> does? I don't know the zram code, but the original looks like it makes >>>> sense but in this one it looks like the locks are duplicative. >>>> >>>> Is the down_read() in the original code be changed to down_write()? >>>> >>> >>> I'm not touching locking around existing READ/WRITE commands. >>> >> >> Your patch does change the locking because now instead of taking the >> zram lock once it takes it and then drops it and then retakes it. This >> looks potentially racy to me but I don't know the code so I will defer >> to any zram maintainer. > > You're right. Nothing prevents zram_slot_free_notify() to repopulate the > free slot queue while we drop the lock. > > Actually, the original code is already racy. handle_pending_slot_free() > modifies zram->table while holding only a read lock. It needs to hold a > write lock to do that. Using down_write for all requests would obviously > fix that, but at the cost of read performance. Now I think we can drop the call to handle_pending_slot_free() in zram_bvec_rw() altogether. As long as the write lock is held when handle_pending_slot_free() is called, there is no race. It's no different from any write request and the current code handles R/W concurrency already. Jerome > >> >> 1) You haven't given us any performance numbers so it's not clear if the >> locking is even a problem. >> >> 2) The v2 patch introduces an obvious deadlock in zram_slot_free() >> because now we take the rw_lock twice. Fix your testing to catch >> this kind of bug next time. >> >> 3) Explain why it is safe to test zram->slot_free_rq when we are not >> holding the lock. I think it is unsafe. I don't want to even think >> about it without the numbers. >> >> regards, >> dan carpenter >> > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
