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Mon, 29 Mar 2021 23:28:01 -0700 (PDT) MIME-Version: 1.0 References: <1616868399-82848-1-git-send-email-guoren@kernel.org> <1616868399-82848-4-git-send-email-guoren@kernel.org> In-Reply-To: From: Guo Ren Date: Tue, 30 Mar 2021 14:27:50 +0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32 To: Anup Patel Cc: Peter Zijlstra , linux-riscv , Linux Kernel Mailing List , "linux-csky@vger.kernel.org" , linux-arch , Guo Ren , Will Deacon , Ingo Molnar , Waiman Long , Arnd Bergmann , Anup Patel X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210330_072805_969138_08E45B96 X-CRM114-Status: GOOD ( 25.18 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Tue, Mar 30, 2021 at 12:54 PM Anup Patel wrote: > > > > > -----Original Message----- > > From: Guo Ren > > Sent: 30 March 2021 08:44 > > To: Peter Zijlstra > > Cc: linux-riscv ; Linux Kernel Mailing List > > ; linux-csky@vger.kernel.org; linux-arch > > ; Guo Ren ; Will > > Deacon ; Ingo Molnar ; Waiman > > Long ; Arnd Bergmann ; Anup > > Patel > > Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add > > ARCH_USE_QUEUED_SPINLOCKS_XCHG32 > > > > On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra > > wrote: > > > > > > On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote: > > > > u32 a = 0x55aa66bb; > > > > u16 *ptr = &a; > > > > > > > > CPU0 CPU1 > > > > ========= ========= > > > > xchg16(ptr, new) while(1) > > > > WRITE_ONCE(*(ptr + 1), x); > > > > > > > > When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock. > > > > > > Then I think your LL/SC is broken. > > > > > > That also means you really don't want to build super complex locking > > > primitives on top, because that live-lock will percolate through. > > Do you mean the below implementation has live-lock risk? > > +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) > > +{ > > + u32 old, new, val = atomic_read(&lock->val); > > + > > + for (;;) { > > + new = (val & _Q_LOCKED_PENDING_MASK) | tail; > > + old = atomic_cmpxchg(&lock->val, val, new); > > + if (old == val) > > + break; > > + > > + val = old; > > + } > > + return old; > > +} > > > > > > > > > > Step 1 would be to get your architecute fixed such that it can provide > > > fwd progress guarantees for LL/SC. Otherwise there's absolutely no > > > point in building complex systems with it. > > > > Quote Waiman's comment [1] on xchg16 optimization: > > > > "This optimization is needed to make the qspinlock achieve performance > > parity with ticket spinlock at light load." > > > > [1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email- > > Waiman.Long@hp.com/ > > > > So for a non-xhg16 machine: > > - ticket-lock for small numbers of CPUs > > - qspinlock for large numbers of CPUs > > > > Okay, I'll put all of them into the next patch > > I would suggest to have separate Kconfig opitons for ticket spinlock > in Linux RISC-V which will be disabled by default. This means Linux > RISC-V will use qspinlock by default and use ticket spinlock only when > ticket spinlock kconfig is enabled. OK > > Regards, > Anup -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/ _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv