Re: [PATCH] ipc/sem.c: Fix complex_count vs. simple op race

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Sat, 02 Jan 2016, Manfred Spraul wrote:

Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a race:

sem_lock has a fast path that allows parallel simple operations.
There are two reasons why a simple operation cannot run in parallel:
- a non-simple operations is ongoing (sma->sem_perm.lock held)
- a complex operation is sleeping (sma->complex_count != 0)

As both facts are stored independently, a thread can bypass the current
checks by sleeping in the right positions. See below for more details
(or kernel bugzilla 105651).

The patch fixes that by creating one variable (complex_mode)
that tracks both reasons why parallel operations are not possible.

The patch also updates stale documentation regarding the locking.

With regards to stable kernels:
The patch is required for all kernels that include the commit 6d07b68ce16a
("ipc/sem.c: optimize sem_lock()") (3.10?)

The alternative is to revert the patch that introduced the race.

I am just now catching up with this, but a quick thought is that we probably
want to keep 6d07b68ce16a as waiting on unlocking all sem->lock should be
much worse for performance than keeping track of the complex 'mode'. Specially
if we have a large array.

Also, any idea what workload exposed this race? Anyway, will take a closer look
at the patch/issue.

Thanks,
Davidlohr
--
To unsubscribe from this list: send the line "unsubscribe stable" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html



[Index of Archives]     [Linux Kernel]     [Kernel Development Newbies]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite Hiking]     [Linux Kernel]     [Linux SCSI]