----- On Jan 12, 2022, at 10:16 AM, Florian Weimer fw@xxxxxxxxxxxxx wrote: > * Mathieu Desnoyers: > >> ----- On Jan 7, 2022, at 2:31 PM, Florian Weimer fw@xxxxxxxxxxxxx wrote: >> >>> * Mathieu Desnoyers: >>> >>>> Allow rseq critical section abort handlers to optionally figure out at >>>> which instruction pointer the rseq critical section was aborted. >>>> >>>> This allows implementing rseq critical sections containing loops, in >>>> which case the commit side-effect cannot be the last instruction. This >>>> is useful to implement adaptative mutexes aware of preemption in >>>> user-space. (see [1]) >>> >>> Could you write the program counter to the rseq area instead? This >>> would avoid discussing which register to clobber. >> >> Using the rseq area for that purpose would be problematic for nested signal >> handlers with rseq critical sections. If a signal happens to be delivered >> right after the abort ip adjustment, its signal handler containing a rseq >> critical section could overwrite the relevant "abort-at-ip" field in the >> rseq per-thread area before it has been read by the abort handler interrupted >> by the signal. >> >> Making this architecture-agnostic is indeed a laudable goal, but I don't >> think the rseq per-thread area is a good fit for this. >> >> I also though about making the clobbered register configurable on a >> per-critical-section basis, but I rather think that it would be >> overengineered: too much complexity for the gain. Unless there are >> very strong reasons for choosing one register over another on a per >> use-case basis ? > > You could perhaps push a signal frame onto the stack. It's going to > be expensive, but it's already in the context switch path, so maybe it > does not matter. The route I'm taking in my subsequent version of the patch is very close to pushing a signal frame: on abort, skip the redzone, and push the abort-at-ip pointer. Then abort handler is then expected to pop the abort-at-ip pointer and unskip the redzone. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
