On 9/8/21 15:05, Jesper Dangaard Brouer wrote: > > > On 08/09/2021 04.54, Andrew Morton wrote: >> From: Vlastimil Babka <vbabka@xxxxxxx> >> Subject: mm, slub: protect put_cpu_partial() with disabled irqs instead of cmpxchg >> >> Jann Horn reported [1] the following theoretically possible race: >> >> task A: put_cpu_partial() calls preempt_disable() >> task A: oldpage = this_cpu_read(s->cpu_slab->partial) >> interrupt: kfree() reaches unfreeze_partials() and discards the page >> task B (on another CPU): reallocates page as page cache >> task A: reads page->pages and page->pobjects, which are actually >> halves of the pointer page->lru.prev >> task B (on another CPU): frees page >> interrupt: allocates page as SLUB page and places it on the percpu partial list >> task A: this_cpu_cmpxchg() succeeds >> >> which would cause page->pages and page->pobjects to end up containing >> halves of pointers that would then influence when put_cpu_partial() >> happens and show up in root-only sysfs files. Maybe that's acceptable, >> I don't know. But there should probably at least be a comment for now >> to point out that we're reading union fields of a page that might be >> in a completely different state. >> >> Additionally, the this_cpu_cmpxchg() approach in put_cpu_partial() is only >> safe against s->cpu_slab->partial manipulation in ___slab_alloc() if the >> latter disables irqs, otherwise a __slab_free() in an irq handler could >> call put_cpu_partial() in the middle of ___slab_alloc() manipulating >> ->partial and corrupt it. This becomes an issue on RT after a local_lock >> is introduced in later patch. The fix means taking the local_lock also in >> put_cpu_partial() on RT. >> >> After debugging this issue, Mike Galbraith suggested [2] that to avoid >> different locking schemes on RT and !RT, we can just protect >> put_cpu_partial() with disabled irqs (to be converted to >> local_lock_irqsave() later) everywhere. This should be acceptable as it's >> not a fast path, and moving the actual partial unfreezing outside of the >> irq disabled section makes it short, and with the retry loop gone the code >> can be also simplified. In addition, the race reported by Jann should no >> longer be possible. > > Based on my microbench[0] measurement changing preempt_disable to > local_irq_save will cost us 11 cycles (TSC). I'm not against the > change, I just want people to keep this in mind. OK, but this is not a fast path for every allocation/free, so it gets amortized. Also it eliminates a this_cpu_cmpxchg loop, and I'd expect cmpxchg to be expensive too? > On my E5-1650 v4 @ 3.60GHz: > - preempt_disable(+enable) cost: 11 cycles(tsc) 3.161 ns > - local_irq_save (+restore) cost: 22 cycles(tsc) 6.331 ns > > Notice the non-save/restore variant is superfast: > - local_irq_disable(+enable) cost: 6 cycles(tsc) 1.844 ns It actually surprises me that it's that cheap, and would have expected changing the irq state would be the costly part, not the saving/restoring. Incidentally, would you know what's the cost of save+restore when the irqs are already disabled, so it's effectively a no-op? Thanks, Vlastimil
