diff options
| author | Thomas Gleixner <tglx@linutronix.de> | 2012-02-22 12:03:30 +0100 |
|---|---|---|
| committer | Clark Williams <williams@redhat.com> | 2012-03-02 11:52:33 -0600 |
| commit | e39ea1d481d16fdc624a229c5db3be49eb1cbf92 (patch) | |
| tree | de175a2200ccdbf2431f32c27916612baf2fe8ae | |
| parent | 21491ae4cc2dc3519f542f0a2e88b5f284545c7e (diff) | |
seqlock: Prevent rt starvation
If a low prio writer gets preempted while holding the seqlock write
locked, a high prio reader spins forever on RT.
To prevent this let the reader grab the spinlock, so it blocks and
eventually boosts the writer. This way the writer can proceed and
endless spinning is prevented.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org
| -rw-r--r-- | include/linux/seqlock.h | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index b44048da105a..723274dc6ea6 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -177,10 +177,33 @@ typedef struct { /* * Read side functions for starting and finalizing a read side section. */ +#ifndef CONFIG_PREEMPT_RT static inline unsigned read_seqbegin(const seqlock_t *sl) { return read_seqcount_begin(&sl->seqcount); } +#else +/* + * Starvation safe read side for RT + */ +static inline unsigned read_seqbegin(seqlock_t *sl) +{ + unsigned ret; + +repeat: + ret = sl->seqcount.sequence; + if (unlikely(ret & 1)) { + /* + * Take the lock and let the writer proceed (i.e. evtl + * boost it), otherwise we could loop here forever. + */ + spin_lock(&sl->lock); + spin_unlock(&sl->lock); + goto repeat; + } + return ret; +} +#endif static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) { |