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Add support for rw semaphore under PREEMPT_RT_FULL

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The main complication from the RT patch set is that the RW semaphore
locks change such that read locks on an rwsem can be taken only by
a single thread.  All other threads are locked out. This single
thread can take a read lock multiple times though. The underlying
implementation changes to a mutex with an additional read_depth
count.

The implementation can be best understood by inspecting the RT
patch.  rwsem_rt.h and rt.c give the best insight into how RT
rwsem works. My implementation for rwsem_tryupgrade is basically
an inversion of rt_downgrade_write found in rt.c. Please see the
comments in the code.

Unfortunately, I have to drop SPLAT rwlock test4 completely as this
test tries to take multiple locks from different threads, which RT
rwsems do not support.  Otherwise SPLAT, zconfig.sh, zpios-sanity.sh
and zfs-tests.sh pass on my Debian-testing VM with the kernel
linux-image-4.8.0-1-rt-amd64.

Tested-by: kernelOfTruth <kerneloftruth@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>
Closes zfsonlinux/zfs#5491
Closes #589
Closes #308
This commit is contained in:
Clemens Fruhwirth
2016-12-17 17:09:57 +01:00
committed by Brian Behlendorf
parent 6d064f7a07
commit 8e99d66b05
3 changed files with 62 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files
module/spl/spl-rwlock.c
+31 -1
View File
@@ -32,7 +32,37 @@
#define DEBUG_SUBSYSTEM S_RWLOCK
#if defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
#if defined(CONFIG_PREEMPT_RT_FULL)
#include <linux/rtmutex.h>
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
ASSERT(rt_mutex_owner(&rwsem->lock) == current);
/*
* Under the realtime patch series, rwsem is implemented as a
* single mutex held by readers and writers alike. However,
* this implementation would prevent a thread from taking a
* read lock twice, as the mutex would already be locked on
* the second attempt. Therefore the implementation allows a
* single thread to take a rwsem as read lock multiple times
* tracking that nesting as read_depth counter.
*/
if (rwsem->read_depth <= 1) {
/*
* In case, the current thread has not taken the lock
* more than once as read lock, we can allow an
* upgrade to a write lock. rwsem_rt.h implements
* write locks as read_depth == 0.
*/
rwsem->read_depth = 0;
return (1);
}
return (0);
}
#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{