* Steven Rostedt (rostedt@xxxxxxxxxxx) wrote: > > On Fri, 7 Nov 2008, Mathieu Desnoyers wrote: > > > > __m_cnt_hi > > is read before > > mmio cnt_lo read > > > > for the detailed reasons explained in my previous discussion with > > Nicolas here : > > http://lkml.org/lkml/2008/10/21/1 > > > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could > > be required so it works also on UP systems safely wrt interrupts). > > smp_rmb turns into a compiler barrier on UP and should prevent the below > description. > Ah, right, preserving program order on UP should be enough. smp_rmb() then. Thanks, Mathieu > -- Steve > > > > > The write side is between the hardware counter, which is assumed to > > increment monotonically between each read, and the value __m_cnt_hi > > updated by the CPU. I don't see where we could put a wmb() there. > > > > Without barrier, the smp race looks as follow : > > > > > > CPU A B > > read hw cnt low (0xFFFFFFFA) > > read __m_cnt_hi (0x80000000) > > read hw cnt low (0x00000001) > > (wrap detected : > > (s32)(0x80000000 ^ 0x1) < 0) > > write __m_cnt_hi = 0x00000001 > > read __m_cnt_hi (0x00000001) > > return 0x0000000100000001 > > (wrap detected : > > (s32)(0x00000001 ^ 0xFFFFFFFA) < 0) > > write __m_cnt_hi = 0x80000001 > > return 0x80000001FFFFFFFA > > (time jumps) > > > > And UP interrupt race : > > > > Thread context Interrupts > > read hw cnt low (0xFFFFFFFA) > > read __m_cnt_hi (0x80000000) > > read hw cnt low (0x00000001) > > (wrap detected : > > (s32)(0x80000000 ^ 0x1) < 0) > > write __m_cnt_hi = 0x00000001 > > read __m_cnt_hi (0x00000001) > > return 0x0000000100000001 > > (wrap detected : > > (s32)(0x00000001 ^ 0xFFFFFFFA) < 0) > > write __m_cnt_hi = 0x80000001 > > return 0x80000001FFFFFFFA > > (time jumps) > > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
