* 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.
