linux-kernel - Re: [PATCH v8 6/7] apei/ghes: Use unrcu

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Date:   Tue, 11 Oct 2022 16:45:38 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Justin He <Justin.He@....com>
Cc:     Len Brown <lenb@...nel.org>, James Morse <James.Morse@....com>,
        Tony Luck <tony.luck@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Robert Richter <rric@...nel.org>,
        Robert Moore <robert.moore@...el.com>,
        Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Jan Luebbe <jlu@...gutronix.de>,
        Khuong Dinh <khuong@...amperecomputing.com>,
        Kani Toshi <toshi.kani@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "devel@...ica.org" <devel@...ica.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Shuai Xue <xueshuai@...ux.alibaba.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        nd <nd@....com>, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

On Tue, Oct 11, 2022 at 02:32:48PM +0000, Justin He wrote:
> My original purpose is to make it pass the sparse checking.

Then do this pls.

This is a combined diff - do a second patch which does only remove the
smp_wmb(). The smp_wmb() there is not needed as the cmpxchg() already
implies a smp_mb() so there's no need for that separate, explicit one.

But it would be prudent to have it in a separate patch just in case.

Thx.

---
diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
index 85acfc589fb7..fd285bf8d114 100644
--- a/drivers/acpi/apei/ghes.c
+++ b/drivers/acpi/apei/ghes.c
@@ -154,7 +154,7 @@ struct ghes_vendor_record_entry {
 static struct gen_pool *ghes_estatus_pool;
 static unsigned long ghes_estatus_pool_size_request;
 
-static struct ghes_estatus_cache *ghes_estatus_caches[GHES_ESTATUS_CACHES_SIZE];
+static struct ghes_estatus_cache __rcu *ghes_estatus_caches[GHES_ESTATUS_CACHES_SIZE];
 static atomic_t ghes_estatus_cache_alloced;
 
 static int ghes_panic_timeout __read_mostly = 30;
@@ -842,9 +842,7 @@ static void ghes_estatus_cache_add(
 			slot_cache = cache;
 		}
 	}
-	/* new_cache must be put into array after its contents are written */
-	smp_wmb();
-	if (slot != -1 && cmpxchg(ghes_estatus_caches + slot,
+	if (slot != -1 && cmpxchg((struct ghes_estatus_cache ** __force)(ghes_estatus_caches + slot),
 				  slot_cache, new_cache) == slot_cache) {
 		if (slot_cache)
 			call_rcu(&slot_cache->rcu, ghes_estatus_cache_rcu_free);

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette