[libdispatch-dev] Race in dispatch_semaphore_wait(DISPATCH_TIME_NOW)

[Dave Zarzycki]

Hi Dmitri,

This is almost surely a bug in the port to these platforms. Apple’s operating system uses Mach to implement this feature, but Mach is not available on Linux and Solaris.

DaveZ


> On May 21, 2014, at 2:14 AM, Dmitri Shubin <sbn at tbricks.com> wrote:
> 
> Hi!
> 
> In our application we assume that dispatch_semaphore_wait(sema, DISPATCH_TIME_NOW) will never block, but recently we found it waiting on OS semaphore.
> 
> Checking source of libdispatch (we used version 215, but the latest available 339.90.1 seems similar) we found the following scenario that could lead to blocking.
> 
> Thread1:
>   dispatch_semaphore_wait(sema, DISPATCH_TIME_NOW);
> 
> Thread2:
>   dispatch_semaphore_signal(sema);
>   dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
> 
> Initial state: dsema_value = 0, internal OS semaphore count = 0
> 
> 1. Thread 1 enters dispatch_semaphore_wait() and decrements dsema_value -> -1, it's < 0, so going slow path.
> 
> 2. Thread 2 signals semaphore, dsema_value incremented from -1 to 0, also going slow path and signals OS semaphore, incrementing its count from 0 to 1.
> 
> 3. Thread 1 enters DISPATCH_TIME_NOW case in switch in dispatch_semaphore_wait_slow(), but since dsema_value == 0 falls through to DISPATCH_TIME_FOREVER case.
> 
> 4. Thread 2 enters dispatch_semaphore_wait, decrements dsema_value from 0 to -1, goes slow path, waits on OS semaphore, decrements its count back to 0 and exits dispatch_semaphore_wait()
> 
> 5. Thread 1 waits forever on OS semaphore.
> 
> Interesting that simple test that we wrote to expose that race can easily show it on Linux (e.g. need 1-5 runs). More runs are needed to see it on Solaris (~5-10 runs).
> But we failed to make it hung on OS X (10.9.3)
> Is there some changes that are not included in open source libdispatch?
> 
> Please check if our analysis is correct.
> Thanks!
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