+ io-mapping-dont-disable-preempt-on-rt-in-io_mapping_map_atomic_wc.patch added to mm-unstable branch

+ io-mapping-dont-disable-preempt-on-rt-in-io_mapping_map_atomic_wc.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: io-mapping: don't disable preempt on RT in io_mapping_map_atomic_wc().
has been added to the -mm mm-unstable branch.  Its filename is
     io-mapping-dont-disable-preempt-on-rt-in-io_mapping_map_atomic_wc.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/io-mapping-dont-disable-preempt-on-rt-in-io_mapping_map_atomic_wc.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: io-mapping: don't disable preempt on RT in io_mapping_map_atomic_wc().
Date: Fri, 10 Mar 2023 17:29:05 +0100

io_mapping_map_atomic_wc() disables preemption and pagefaults for
historical reasons.  The conversion to io_mapping_map_local_wc(), which
only disables migration, cannot be done wholesale because quite some call
sites need to be updated to accommodate with the changed semantics.

On PREEMPT_RT enabled kernels the io_mapping_map_atomic_wc() semantics are
problematic due to the implicit disabling of preemption which makes it
impossible to acquire 'sleeping' spinlocks within the mapped atomic
sections.

PREEMPT_RT replaces the preempt_disable() with a migrate_disable() for
more than a decade.  It could be argued that this is a justification to do
this unconditionally, but PREEMPT_RT covers only a limited number of
architectures and it disables some functionality which limits the coverage
further.

Limit the replacement to PREEMPT_RT for now.  This is also done
kmap_atomic().

Link: https://lkml.kernel.org/r/20230310162905.O57Pj7hh@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
  Link: https://lore.kernel.org/CAFLxGvw0WMxaMqYqJ5WgvVSbKHq2D2xcXTOgMCpgq9nDC-MWTQ@mail.gmail.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/include/linux/io-mapping.h~io-mapping-dont-disable-preempt-on-rt-in-io_mapping_map_atomic_wc
+++ a/include/linux/io-mapping.h
@@ -69,7 +69,10 @@ io_mapping_map_atomic_wc(struct io_mappi
 
 	BUG_ON(offset >= mapping->size);
 	phys_addr = mapping->base + offset;
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
+	else
+		migrate_disable();
 	pagefault_disable();
 	return __iomap_local_pfn_prot(PHYS_PFN(phys_addr), mapping->prot);
 }
@@ -79,7 +82,10 @@ io_mapping_unmap_atomic(void __iomem *va
 {
 	kunmap_local_indexed((void __force *)vaddr);
 	pagefault_enable();
-	preempt_enable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+	else
+		migrate_enable();
 }
 
 static inline void __iomem *
@@ -162,7 +168,10 @@ static inline void __iomem *
 io_mapping_map_atomic_wc(struct io_mapping *mapping,
 			 unsigned long offset)
 {
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
+	else
+		migrate_disable();
 	pagefault_disable();
 	return io_mapping_map_wc(mapping, offset, PAGE_SIZE);
 }
@@ -172,7 +181,10 @@ io_mapping_unmap_atomic(void __iomem *va
 {
 	io_mapping_unmap(vaddr);
 	pagefault_enable();
-	preempt_enable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+	else
+		migrate_enable();
 }
 
 static inline void __iomem *
_

Patches currently in -mm which might be from bigeasy@linutronix.de are

io-mapping-dont-disable-preempt-on-rt-in-io_mapping_map_atomic_wc.patch