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a988a35a93
The comment above the Linux 3.16 kernel's clear_bit() states:
/**
* clear_bit - Clears a bit in memory
* @nr: Bit to clear
* @addr: Address to start counting from
*
* clear_bit() is atomic and may not be reordered. However, it does
* not contain a memory barrier, so if it is used for locking purposes,
* you should call smp_mb__before_atomic() and/or smp_mb__after_atomic()
* in order to ensure changes are visible on other processors.
*/
This comment does not make sense in the context of x86 because x86 maps the
operations to barrier(), which is a compiler barrier. However, it does make
sense to me when I consider architectures that reorder around atomic
instructions. In such situations, a processor is allowed to execute the
wake_up_bit() before clear_bit() and we have a race. There are a few
architectures that suffer from this issue.
In such situations, the other processor would wake-up, see the bit is still
taken and go to sleep, while the one responsible for waking it up will
assume that it did its job and continue.
This patch implements a wrapper that maps smp_mb__{before,after}_atomic() to
smp_mb__{before,after}_clear_bit() on older kernels and changes our code to
leverage it in a manner consistent with the mainline kernel.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
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| .. | ||
| Makefile.in | ||
| spl-atomic.c | ||
| spl-condvar.c | ||
| spl-cred.c | ||
| spl-err.c | ||
| spl-generic.c | ||
| spl-kmem-cache.c | ||
| spl-kmem.c | ||
| spl-kobj.c | ||
| spl-kstat.c | ||
| spl-mutex.c | ||
| spl-proc.c | ||
| spl-rwlock.c | ||
| spl-taskq.c | ||
| spl-thread.c | ||
| spl-tsd.c | ||
| spl-vmem.c | ||
| spl-vnode.c | ||
| spl-xdr.c | ||
| spl-zlib.c | ||