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Linux 5.2 compat: rw_tryupgrade()

Commit torvalds/linux@46ad084 has removed the architecture specific
rwsem source and headers leaving only the generic version.  As part
of this change the RWSEM_ACTIVE_READ_BIAS and RWSEM_ACTIVE_WRITE_BIAS
macros were moved to the private kernel/locking/rwsem.h header.
This results in a build failure because these macros were required
to implement the rw_tryupgrade() compatibility function.

In practice, this isn't a major problem because there are only a
few consumers of rw_tryupgrade() and because consumers of rw_tryupgrade
should be written to retry using rw_enter(RW_WRITER).

After auditing all of the callers only dmu_zfetch() was determined
not to perform a retry.  It has been updated in this commit to
resolve this issue.

That said, the rw_tryupgrade() functionality should be considered
for possible removal in a future release due to the difficultly
in supporting the interface.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
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behlendorf committed May 8, 2019
1 parent 3b77084 commit 9973f682a99bc5e05e6b3abcbda5239b01d1660f
Showing with 20 additions and 5 deletions.
  1. +1 −1 include/spl/sys/rwlock.h
  2. +9 −1 module/spl/spl-rwlock.c
  3. +10 −3 module/zfs/dmu_zfetch.c
@@ -36,7 +36,7 @@
#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
#define SPL_RWSEM_SINGLE_READER_VALUE (1)
#define SPL_RWSEM_SINGLE_WRITER_VALUE (-1)
#else
#elif defined(RWSEM_ACTIVE_MASK)
#define SPL_RWSEM_SINGLE_READER_VALUE (RWSEM_ACTIVE_READ_BIAS)
#define SPL_RWSEM_SINGLE_WRITER_VALUE (RWSEM_ACTIVE_WRITE_BIAS)
#endif
@@ -85,7 +85,8 @@ __rwsem_tryupgrade(struct rw_semaphore *rwsem)
spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
return (ret);
}
#elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
#elif defined(RWSEM_ACTIVE_MASK)
#if defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
@@ -104,6 +105,13 @@ __rwsem_tryupgrade(struct rw_semaphore *rwsem)
return (val == SPL_RWSEM_SINGLE_READER_VALUE);
}
#endif
#else
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
return (0);
}
#endif

int
rwsem_tryupgrade(struct rw_semaphore *rwsem)
@@ -214,6 +214,7 @@ dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data)
uint64_t end_of_access_blkid;
end_of_access_blkid = blkid + nblks;
spa_t *spa = zf->zf_dnode->dn_objset->os_spa;
krw_t rw = RW_READER;

if (zfs_prefetch_disable)
return;
@@ -234,7 +235,8 @@ dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data)
if (blkid == 0)
return;

rw_enter(&zf->zf_rwlock, RW_READER);
retry:
rw_enter(&zf->zf_rwlock, rw);

/*
* Find matching prefetch stream. Depending on whether the accesses
@@ -272,8 +274,13 @@ dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data)
* a new stream for it.
*/
ZFETCHSTAT_BUMP(zfetchstat_misses);
if (rw_tryupgrade(&zf->zf_rwlock))
dmu_zfetch_stream_create(zf, end_of_access_blkid);
if (rw == RW_READER && !rw_tryupgrade(&zf->zf_rwlock)) {
rw_exit(&zf->zf_rwlock);
rw = RW_WRITER;
goto retry;
}

dmu_zfetch_stream_create(zf, end_of_access_blkid);
rw_exit(&zf->zf_rwlock);
return;
}

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