Am 29.10.2010 10:27, Philippe Gerum wrote: > On Fri, 2010-10-29 at 09:00 +0200, Jan Kiszka wrote: >> Am 28.10.2010 21:34, Philippe Gerum wrote: >>> On Thu, 2010-10-28 at 21:15 +0200, Gilles Chanteperdrix wrote: >>>> Jan Kiszka wrote: >>>>> Gilles, >>>>> >>>>> I happened to come across rthal_mark_irq_disabled/enabled on arm. On >>>>> first glance, it looks like these helpers manipulate irq_desc::status >>>>> non-atomically, i.e. without holding irq_desc::lock. Isn't this fragile? >>>> >>>> I have no idea. How do the other architectures do? As far as I know, >>>> this code has been copied from there. >>> >>> Other archs do the same, simply because once an irq is managed by the >>> hal, it may not be shared in any way with the regular kernel. So locking >>> is pointless. >> >> Indeed, I missed that all the other archs have this uninlined in hal.c. >> >> However, this leaves at least a race between xnintr_disable/enable and >> XN_ISR_PROPAGATE (ie. the related Linux path) behind. > > I can't see why XN_ISR_PROPAGATE would be involved here. This service > pends an interrupt in the pipeline log.
And this finally lets Linux code run that fiddles with irq_desc::status as well - potentially in parallel to an unsyncrhonized xnintr_irq_disable in a different context. That's the problem. > >> Not sure if it >> matters practically - but risking silent breakage for this micro >> optimization? > > It was not meant as an optimization; we may not grab the linux > descriptor lock in this context because we may enter it in primary mode. Oh, that lock isn't harden as I somehow assumed. This of course complicates things. > >> Is disabling/enabling really in the highly >> latency-critical anywhere? Otherwise, I would suggest to just plug this >> by adding the intended lock for this field. > > The caller is expected to manage locking; AFAICS the only one who does > not is the RTAI skin, which is obsolete and removed in 2.6.x, so no big > deal. The problem is that IRQ forwarding to Linux may let this manipulation race with plain Linux code, thus has to synchronize with it. It is a corner case (no one is supposed to pass IRQs down blindly anyway - if at all), but it should at least be documented ("Don't use disable/enable together with IRQ forwarding unless you acquire the descriptor lock properly!"). BTW, do we need to track the descriptor state in primary mode at all? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux _______________________________________________ Xenomai-core mailing list Xenomaifirstname.lastname@example.org https://mail.gna.org/listinfo/xenomai-core