On Wed, 31 Jul 2019, Jan Kara wrote: > On Tue 30-07-19 13:24:56, Thomas Gleixner wrote: > > Bit spinlocks are problematic if PREEMPT_RT is enabled. They disable > > preemption, which is undesired for latency reasons and breaks when regular > > spinlocks are taken within the bit_spinlock locked region because regular > > spinlocks are converted to 'sleeping spinlocks' on RT. > > > > Substitute the BH_State and BH_JournalHead bit spinlocks with regular > > spinlock for PREEMPT_RT enabled kernels. > > Is there a real need for substitution for BH_JournalHead bit spinlock? The > critical sections are pretty tiny, all located within fs/jbd2/journal.c. > Maybe only the one around __journal_remove_journal_head() would need a bit > of refactoring so that journal_free_journal_head() doesn't get called > under the bit-spinlock. Makes sense. > BH_State lock is definitely worth it. In fact, if you placed the spinlock > inside struct journal_head (which is the structure whose members are in > fact protected by it), I'd be even fine with just using the spinlock always > instead of the bit spinlock. journal_head is pretty big anyway (and there's > even 4-byte hole in it for 64-bit archs) and these structures are pretty > rare (only for actively changed metadata buffers). Just need to figure out what to do with the ASSERT_JH(state_is_locked) case for UP. Perhaps just return true for UP && !DEBUG_SPINLOCK? Thanks, tglx
